The vast majority of semiconductor devices such as diodes, transistors and the like utilize a body of crystalline semiconductor material such as silicon and germanium, principally in their single crystal form. Single crystal silicon, though very costly, is the primary semiconductor material for the electronic device industry. Single crystal semiconductor materials such as silicon display a highly symmetrical well-ordered atomic structure resulting in favorable basic electronic properties.
A collective description of the interrelationship of several of the basic properties is referred to in the art as the semiconductor's transport properties. Transport properties generally define the ability of the semiconductor material to move a generated or injected charge carrier through the material. One of the more important components of a material's transport properties is the mobility of the charge carriers. Broadly defined, the mobility determines the rate at which an electron (or hole) will migrate through the semiconductor material under the influence of a given electric field. More particularly, it is the velocity with which the carrier (hole or electron) moves through the material per unit applied field, expressable as .nu.=.mu.E where .nu. is the velocity, .mu. is the mobility, and E is the electric field. Embodied in a semiconductor device such as a field effect transistor, the mobility is an important factor in determining the switching time of the device.
In the well-ordered atomic structure of single crystal silicon or similar such material, a generated or injected charge carrier will travel rapidly through the material, displaying majority carrier mobility as high as 1,000 cm.sup.2 /v-sec. These crystalline materials are generally free of localized trapping states in the energy gap, resulting in an uninterfered transit of the charge carriers through the material. If a planar distribution of charge carriers is injected into the aforedescribed high mobility semiconductor, these charge carriers would (under the influence of an electric field) drift uniformly through the semiconductor in a coherent Gaussian packet, which packet would be broadened only by diffusive spreading of the injected charge packet. Furthermore, the time required for the center of said charge packet to travel a specified distance is inversely proportional to the field in the material, provided the field is uniform. This coherent transport of charge carriers, undominated by localized states within the semiconductor, is conventionally referred to as non-dispersive transport. As those of the art readily recognize, semiconductor materials in their natural state do not typically display such favorable electronic characteristics. In the case of single crystals, silicon and germanium required extensive research before the processing techniques produced such an electronically favorable semiconductor material. Those of the art will further recognize that the property influencing techniques for altering semiconductor materials are unique to each individual material.
A relatively new and pioneering field of the semiconductor industry is amorphous semiconductor materials. Unlike their ordered single-crystal counterparts, amorphous semiconductor materials display no long range order. This intrinsic lack of order had conventionally been considered severely detrimental to the electronic transport properties and in particular to mobility of charge carriers. Those of the art had initially considered this a fundamental obstacle in using such materials ln semiconductor applications. Amorphous semiconductor materials normally display a substantial density of localized states in the energy gap. These states are conventionally divided into two categories depending on their location in the energy gap and their resultant effect on the transport properties of the semiconductor materials. A first category may be defined as shallow states (less than about 0.2 ev from the band edge) which exhibits relatively short trapping time (less than or equal to about 10.sup.-9 seconds). These shallow states are generally attributed to fluctuations in the local potentials. A second category may be generally referred to as deep states. These states are typically greater than about 0.2 ev from the band edge and trap charge carriers for a relatively long period of time (greater than 10.sup.-9 sec.). The term trapping, as known to those in the art, refers to the influence of the localized states upon a charge carrier. Deep states are generally attributed to impurities or gross defects in the amorphous semiconductor lattice such as unsatisfied or dangling bonds.
Since trapping and detrapping events in shallow states are short relative to a charge carrier transit time over distances likely to be involved in a semiconductor device (greater than 1 micron and less than 100 microns), their general effect is to produce only a net lowering of the mobility and will not alter otherwise non-dispersive transport properties of a semiconductor material. Deep states, however, display trapping and detrapping events which substantially alter the transport properties of a semiconductor material. These deep states cause the aforedescribed packet of uniformly injected charge carriers to spread out or disperse during their transit across the semiconductor material. This phenomena arises from charge carriers remaining in deep states while other charge carriers transit the entire distance of the semiconductor device. Thus, the injected charge carriers end up being dispersed throughout the semiconductor material before reaching the opposite electrode.
Graphically illustrating the aforedescribed properties, FIG. 3 is a conventional representation of results for a time of flight experiment, a known technique for evaluating semiconductor transport properties.
Trace 52 shows the collection of charges with respect to time in a semiconductor material having dispersive transport properties. Dispersive transport properties produce polarization effects, long time constants, and similarly undesirable electronic effects in semiconductor devices.
To more clearly illustrate, assume that a slab of amorphous semiconductor material has electrodes deposited on the two opposing faces. A uniform field is impressed across the material between the two electrodes and a packet of charge carriers is injected into the surface of the semiconductor near one electrode. As the carriers drift across the film, a current is induced in the external or monitoring circuit. As the carriers are collected at the opposing electrode, the current in the external circuit falls off. For a semiconductor material having dispersive transport properties, this fall off of current will be protracted, as illustrated in FIG. 3 as trace 52. In contrast, a semiconductor material having non-dispersive transport properties displays an abrupt drop in the current as the carriers are collected at the opposing electrode, illustrated in FIG. 3 as trace 50.
From the length of time it takes the carriers to reach the back electrode, more commonly referred to in the art as the transit time, one can calculate the carrier mobility utilizing the relationship: ##EQU1## where .mu. is the drift mobility, hereinafter mobility;
d is the semiconductor thickness; PA1 V is the voltage impressed across the electrodes; and PA1 t is the transit time. PA1 1. A. Moore, Applied Physics Letters, 31, 762 (1977) PA1 2. W. Fuhs, M. Milleville and J. Stuke, Physica Status Solidi, 89, 495 (1978) PA1 3. W. Spear and P. LeComber, Journal of Non-Crystalline Solids, 8-10, 727 (1972)
For disordered materials such as amorphous silicon, which normally exhibit a significant amount of dispersion in the carrier transport, those of the art identify the time of arrival of the leading edge of the charge packet at the opposing electrode as the transit time. This time is indicated in FIG. 3 as t.sub.e '. Although this transit time effectively over-estimates the carrier mobility, operationally it is normally the only time that can be identified in dispersive materials which show no well defined fall-off in current. In some cases even the first arrival time of carriers in dispersive materials may not be detectable on a linear scale as is illustrated by trace 52 of FIG. 3. In this situation, those of the art identify the first arrival time from a plot of the log of the current as a function of the log of the time. In contrast, materials which exhibit non-dispersive transport do not experience this difficulty. Instead of basing the mobility on the arrival of the leading edge of the charge packet, one identifies the transit time as the arrival time of the center of the charge packet, which more accurately reflects the true carrier mobility. This transit time is indicated by t.sub.e in FIG. 3. It is well represented by the time at which the transient current drops to about one half its value in the current plateau following the light flash. This definition of the transit time recognized in the art of non-dispersive semiconductors, will be adopted here. A more thorough discussion of one example of non-dispersive charge carrier transport through semiconductor materials, may be found in Canali et al., Physical Review, B12, 2265 (1975).
The present invention relates to amorphous silicon whose transport properties have been substantially altered to provide non-dispersive transport of charge carriers through the amorphous semiconductor material, making the material suitable for use in diodes, transistors, detectors, solar cells and other similar such semiconductor devices. The amorphous silicon in thin film form is deposited by the known technique of glow discharge decomposition of silane. A judicious selection of deposition parameters is herein shown to produce a substantially unique electronic semiconductor material having the aforedescribed nondispersive transport properties and relatively high majority carrier mobility.
Prior to the present invention, all amorphous materials with the exception of amorphous selenium and possibly silicon dioxide exhibit dispersive transport at room temperature. See, for example, Pfister & Scher, Physical Review, B15, 2062 (1977); Pai, Journal of Chemical Physics, 52, 2285 (1970). Tetrahedrally coordinated amorphous network materials such as amorphous silicon, for example, are well known to display dispersive transport of charge carriers; see, for example, Allan et al., Proceedings of Edinburgh Conference, 1977, Scher & Montroll, Physical Review, B12, 2455 (1975). In contrast, the present invention teaches deposition techniques which produce an unexpected alteration of the conventionally observed transport properties in amorphous silicon.